The present invention relates to a silica optical fiber, particularly to a silica optical fiber superior in resistance to high energy.
In recent years, an optical fiber scope is frequently used for observation and checkup of places to which human or a camera cannot have access, such as a nuclear power plant, inside of a blast furnace, a boiler of power plants and the like. An optical fiber scope generally consists of an image fiber, an objective lens, an eyepiece, a light guide, and the like.
The image fiber consists of from thousands to tens of thousands of pixel fibers integrated by alignment, and transmits images formed on a fiber end by the lens to the other end upon decomposition of the images into each pixel fiber. The image fiber includes a multiple type fiber and a bundle type fiber. The multiple type fiber includes a number of aligned pixel fibers that are integrated by melting and drawing, thus forming a common clad. In contrast, the bundle type fiber includes a number of constituent fibers that are fixed by adhesion on both ends.
The optical fiber scope is used under the above-mentioned severe conditions. Particularly in a nuclear power plant where a high-energy electromagnetic wave irradiation of xcex3-ray is inevitable, pure silica is generally used as the core material of an image fiber, because it is superior in resistance to high energy. Due to the superior characteristics of the resistance to high energy, moreover, a silica optical fiber is also used for transmitting electromagnetic waves such as UV light and waves having shorter wavelengths such as X-ray.
However, the resistance to high energy of conventional silica optical fibers has not been improved to a sufficient level, and there have been ongoing attempts to improve resistance of silica optical fibers to high energy.
The present inventors have tried to improve optical fibers in the resistance to ultraviolet light by making the core of a silica optical fiber contain F element and OH group and removing Cl element from the core, as disclosed in JP-A-5-147966 (JP-B-8-9489), though sufficient effect has not been achieved.
Based on such results, the present inventors have noted completely different effects provided by the F element and the Cl element in radiation resistance and resistance to UV light, despite the fact that these elements fall under the seventh group in the periodic table, and acquired a completely new conception that the period of the element contained in the pure silica core material may be deeply involved in the resistance to high energy.
It is therefore an object of the present invention to provide a silica optical fiber superior in radiation resistance and having resistance to high energy of the light in the ultraviolet region, particularly to the light in the shorter wavelength ultraviolet region, specifically in an X-ray region.
The present invention is based on the finding that a pure-silica containing Cl element is markedly inferior to that containing F element in the resistance to high energy of ultraviolet radiation and xcex3-ray radiation because, in addition to the difference from Si element in the electronegativity and the difference in the atomic radius from the Si element or oxygen (O) element, the Cl element basically has a 3d orbital or 4s orbital that permits electrons in the ground state to easily transit to the 3d or 4s orbital upon application of an energy, thus producing an excited state, and easily becomes a valence state such as trivalence, pentavalence and heptavalence, producing many radicals. From this finding, the present inventors have considered that the changes in valence due to high energy irradiation may cause a greater effect on the high energy resistance of quartz by not only the F element and the Cl element, but by the elements belonging to the first period-second period and non-metallic elements belonging to the third period-seventh period.
Further studies based on the above-mentioned consideration have led to the following conclusion. That is, the major differences between the elements belonging to the third period-seventh period, as represented by the Cl element, and the elements belonging to the first period-second period, as represented by the F element, are that the elements belonging to the third period-seventh period have any of the factors of (1) a 3d orbital or 4s orbital that permits electron(s) in the ground state to easily transit to the 3d or 4s orbital upon application of a large external energy, such as xcex3-ray and UV light, thus producing an excited state, (2) a small difference in the electronegativity from Si element, and (3) a greater difference in atomic radius from Si element or oxygen element. Because they possess any of these elements (1)-(3), the elements belonging to the third period-the seventh period are considered to be easily activated when subjected to a large external energy and easily destroy a quartz structure, as compared to the elements belonging to the first period-second period.
Because the elements belonging to the first period-second period, particularly C element, show an extremely small difference in the atomic radius from the other elements belonging to the first period-second period, when they are introduced into the quartz structure and an Sixe2x80x94C bond or Cxe2x80x94O bond is produced upon supplementation of radicals, the occurrence of the defects of the quartz structure, such as distortion and breakage of bond by a large external energy (e.g., xcex3-ray, UV light and the like), is suppressed, whereby resistance to the high energy of quartz is considered to be remarkably improved.
Based on these new findings, the present invention provides a silica optical fiber characterized by the following.
(1) A silica optical fiber comprising a pure-silica core and a cladding layer formed on the pure-silica core, wherein the pure-silica core comprises a C element and has a content of elements belonging to the third period-the seventh period of the periodic table, except an Si element that constitutes the quartz structure, of not more than 100 ppm.
(2) The silica optical fiber of the above-mentioned (1), wherein the content of the C element is 10 ppm-500 ppm.
(3) The silica optical fiber of the above-mentioned (1), wherein the pure-silica core further comprises an F element and/or an OH group.
(4) The silica optical fiber of above-mentioned (3), wherein the content of the F element and/or the OH group is not more than 5000 ppm.
The present invention has been made based on the aforementioned new findings. In the silica optical fiber of the present invention, the content of the elements belonging to the third period-seventh period (except the Si element constituting the quartz structure) in the pure-silica core is set for not more than 100 ppm, and the C element is contained in the pure-silica core, thereby improving the radiation resistance, and the resistance to electromagnetic waves in the ultraviolet region and the region of shorter wavelengths (e.g., X-ray), namely, high energy resistance.
According to the present invention, an F element and/or an OH group are/is preferably contained in the pure-silica core. When, for example, an F element is contained, it reacts with radicals to produce a relatively stable chemical structure such as Sixe2x80x94F. When an OH group is contained, it reacts with radicals to produce a relatively stable chemical structure such as Sixe2x80x94OH. When the F element and/or OH group are/is contained, therefore, the high energy resistance can be further improved due to the mutual action with the aforementioned stable structure provided by the introduction of the C element.
When one of the OH group and F element is to be contained along with the C element, it is preferably the F element, which becomes more effective when combined to the C element. In particular, when the radiation resistance at a wavelength shorter than the visible region is desired, an OH group does not need to be contained. The F element can be contained by using a fluorine compound such as SiF4, CF4, C2F6, BF3 and the like, which are free of an element of the third period other than Si element, relative to the silicon compound, which is a main starting material of the quartz structure to be mentioned later. When a fluorine compound containing an element belonging to the third period-seventh period, such as SF6 and PF6, is used, the content of the elements belonging to the third period-seventh period is increased, which is not preferable.
The method for producing the silica optical fiber of the present invention is explained in the following. The silica optical fiber of the present invention can be obtained by, for example, preparing a preform to be the base material and fiber-drawing this preform. The preform can be fiber-drawn by a known method comprising softening a preform by heating and drawing it. The preform can be obtained by, for example, forming a doped-silica glass to be a cladding layer on a pure-silica glass rod to be a core, or by inserting a pure-silica glass rod to be a core into a tube in which a doped-silica glass to be a cladding layer has been formed, and preferably removing partially or entirely the outermost tube layer by, for example, a fire polishing method.
The pure-silica glass rod (pure-silica core rod) to be the core of a silica optical fiber can be prepared according to the VAD method by, for example, hydrolyzing a silicon compound (the main material) with oxygen and hydrogen, depositing a synthetic silica particulate to form a porous silica, and heat-melting the same for vitrification (dope element can be added during deposition or sintering), or according to the plasma method for direct vitrification of a silicon compound (the main material) and a dopant.
For preparation of the above-mentioned pure-silica glass rod, the silicon compound and dopant, and the method such as the VAD method and plasma method should be selected in such a manner that the content of the element belonging to the third period-seventh period is not more than 100 ppm, and a C element, more conveniently an F element and/or an OH group are/is contained. Specifically, when tetramethoxysilane (Si(OCH3)4), tetraethoxysilane (Si(OC2H5)4) or methyltrimethoxysilane (CH3Si(OCH3)3) is used as the silicon compound and a fluorine compound such as SiF4, CF4, C2F6 and BF3 is used as the dopant, the F element and the C element can be contained. Moreover, by employing the VAD method that uses an oxyhydrogen flame for heating, the OH group can be contained.
In the present invention, when the content of the C element is too high, the transmittance becomes low and when it is too low, the amount of oxygen in the pure-silica glass network structure becomes in excess to produce xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1 bond in a great amount. As a result, a great absorption by high-energy irradiation is caused, lowering the radiation resistance. Therefore, the content of the C element is preferably 10-500 ppm, more preferably 10-200 ppm. The content of the C element can be controlled by, for example, adjusting the amount of oxygen during deposit, or adjusting the amount of oxygen flown in a temperature elevating state before sintering in a sintering apparatus.
The content of the F element and/or OH group is not more than 5000 ppm, preferably 100-3000 ppm.
When the content of the F element is too high, the refractive index of the core member becomes low, which makes the difference in refraction between the core and the cladding too small, and when it is too low, the effect of the high energy resistance becomes weak. Therefore, the content of the F element (simple substance) is preferably 100-2000 ppm. When the content of the OH group is too high, xe2x89xa1Sixe2x80x94O deficiency occurs by irradiation to make the effect of the high energy resistance weak, and when it is too low, it tends to aggravate the initial transmission property particularly in the ultraviolet region. For a use other than in the ultraviolet region, an OH group does not need to be particularly contained. Therefore, the content of the OH group (simple substance) is preferably set for 0-500 ppm.
The content of the F element and OH group can be controlled by adjusting the amount of the dope element containing an F element, based on the dehydrating action of the F element, wherein a greater content of the F element means a lower content of the OH group.
The content of the C element, F element and OH group can be measured by a known method. In the case of the C element, for example, it can be measured by a combustion-infrared absorption analysis, in the case of the F element, it can be measured by an ion-selective electrode analysis, and in the case of the OH group, it can be measured by the analysis using an infrared spectrometer.
As mentioned above, a preform can be prepared by forming a doped-silica glass cladding layer. The layer of doped-silica glass can be formed by, for example, the CVD method, the MCVD method or the plasma method, using a mixed gas of BCl13, BF3, SiCl4 and oxygen as starting material gases. Alternatively, the layer can be formed using a mixed gas of BCl13, SiF4 and oxygen or a mixed gas of BF3, BCl3, SiF4 and oxygen, as a starting material gas.
The silica optical fiber of the present invention can be used as a single mode fiber or a multimode fiber. In addition, the silica optical fiber of the present invention can be used as a quartz bundle including bundled silica optical fibers or as a fiber constituting an image fiber.